Change the way the Hamiltonian is specified

[mortele]

The way the Hamiltonian form is specified is not scalable. We should consider changing it to where something like this is a valid specification:

hamiltonian = [
    "chi-interaction", 
    "kappa-incompressibility",
]

or

hamiltonian = [
    "phi-squared", 
]

or

hamiltonian = [
    "chi-interaction", 
    "carnahan–starling",
]

or

hamiltonian = [
    "chi-interaction", 
    "carnahan–starling",
]

or

hamiltonian = [
    "chi-interaction", 
    "phi-squared",
    "kappa-incompressibility",
]

---

This would allow easy and intuitive combinations of Hamiltonian forms in the long term.